Objective: To compare results obtained from assay of total thyroxine (T4) concentration in serum of dogs and cats by use of 4 methods.

Sample population: Serum samples obtained from 98 dogs and 100 cats and submitted by veterinarians to an endocrine testing laboratory.

Procedure: Total T4 concentration was determined in each sample by use of 4 assay methods. Assay methods included a radioimmunoassay (RIA) marketed for use in dogs, an RIA for use in humans, a chemiluminescent enzyme immunoassay for use in humans, and an in-house ELISA.

Results: Total T4 concentrations obtained by use of all methods were significantly correlated. Bias-plot comparison revealed similar good overall agreement. Total T4 concentrations determined by use of the RIA marketed for use in dogs were generally lower than concentrations measured by use of the other methods. Clinical comparisons were made by evaluation of the T4 results in the context of the reference range recommended by each laboratory. A difference was found for clinical comparisons on the basis of T4 assay method when used to identify dogs as possible hypothyroid suspects. This difference was related more to the reference range used than to the absolute T4 value. The number of hyperthyroid-suspect cats with T4 values greater than the reference range was the same for each of the 4 assay methods.

Conclusions and clinical relevance: Total T4 concentrations determined in dogs and cats by use of 4 commonly used methods provided similar and consistent results.